Venezuela plunged deeper into turmoil Monday as security forces put down a pre-dawn uprising by national guardsmen that triggered violent street protests, and the Supreme Court moved to undercut the opposition-controlled congress' defiant new leadership.

Greenland's enormous ice sheet is melting at such an accelerated rate that it may have reached a "tipping point," and could become a major factor in sea-level rise around the world within two decades, scientists said in a study published Monday.